The molecular structure of human LDH-A and LDH-B mutant genes have been characterized. Human lactate dehydrogenase-A mutant gene was isolated from the genomic DNA library of a patient deficient in LDH-A (muscle) subunit. The nucleotide sequences of seven protein-coding exons were determined and a deletion of 20 base-pairs in exon 6 was found. This mutation results in a frameshift translation and premature termination. The predicted incomplete LDH-A (M) subunit containing only 259 instead of 331 amino acids appears to be degraded rapidly, since no protein was detected immunologically. A human lactate dehydrogenase-B mutant gene was isolated from a genomic DNA library constructed from a patient with unstable LDH-B (heart) subunit. The nucleotide sequences of seven protein-coding exons were determined and a single nucleotide substitution of G by A at Arg condon CGC in exon 4 was found. This mutation results in an amino-acid replacement of a conserved arginine by histidine at the residue "173," which is involved in an anion-binding site at the P-axis of LDH subunits. This information will allow more accurate evaluation of genetic mutation events caused by mutagens and eventually will be of value to improve human health care.